1. Field of the invention
This invention relates to an improvement in a lens position control device which functions to automatically control the focal length of the lens in such a manner as to keep unvarying the ratio of area occupied within a picture plane by an object to be photographed even when a distance to the object changes.
2. Description of the Related Art
Generally, the conventional devices of this kind have been arranged to have the focal length "f", the object distance R and the size X of the area occupied by the object within the picture plane in a relation of "f"=f (R, X). In order to obtain the automatic zooming function, it is thus practiced in most cases to find the object distance R by means of an automatic focusing device or a device for detecting the absolute position of a focusing lens group, to compute the focal length "f" according to the object distance R and a predetermined value of the object size X and then to control the position of a power varying lens group. It is therefore necessary to accurately find the object distance R and the focal length "f" prior to the lens position control.
The above-stated arrangement of the conventional device, therefore, has the following drawbacks:
(1) It requires a complex structural arrangement for detecting an object distance R. (2) A complex arrangement is also required for detecting the focal length "f". (3) A CPU is required to perform complex control in computing the focal length "f" on the basis of the object distance R and the size X of the object within the picture plane.
The lens position control device, therefore, has been desired to have a simpler arrangement for its automatic zooming function. The details of the above-stated drawbacks (1), (2) and (3) are as described below:
FIG. 16 of the accompanying drawings is a sectional view showing the main part of a most typical four-group zoom type photo-taking lens. A lens group 1 (front lens) consists of focusing lenses 1-A, 1-B, and 1-C. A power varying lens group 2 (variator) consists of lenses 2-A, 2-B and 2-C. A lens 3 (compensator) is arranged to act in association with the variator 2. An afocal lens 4 is stationary. Image forming lens groups (relay lenses) 5 and 6 consist of lenses 5-A, 5-B and 5-C and lenses 6-A, 6-B and 6-C, respectively. A front-lens frame 7 is formed in one body with a female helicoid member 8. A reference numeral 13 denotes a stationary lens barrel.
The stationary lens barrel 13 has a helicoid thread formed along the outer circumference thereof. This enables the front lens 1 to be moved forward and backward in the direction of an optical axis when the front-lens frame 7 is rotated either by a drive source such as a motor or the like which is not shown or by a manual operation. With the front lens 1 thus moved, a focusing distance varies accordingly. In other words, a front-lens position adjusting means is formed jointly by the front-lens frame 7, the female helicoid member 8, the stationary lens barrel 13 and a motor or the like which is not shown.
The illustration further includes a lens frame 9 for the variator 2; a cam follower 10; a cam ring 11; a macro-photography operation knob 14; a zoom ring 15; a lens frame 17 for the compensator 3; a guide bar 18 for the lens frames 9 and 17 of the variator 2 and the compensator 3; a spring 19 which is arranged to push the cam ring 11; and a gear part 12 which is interlocked with the motor.
The position of the variator 2 and that of the compensator 3 are changed either by a drive source such as a motor which is not shown or by a manual operation on the zoom ring 15. This changes the focal length of the photo-taking lens. These parts jointly form a means for adjusting the positions of the power varying and compensating lens groups. More specifically, the rotation of the zoom ring 15 causes the rotation of the cam ring 11 which is fitted in the inner circumferential side of the stationary lens barrel 13. Then, the cam follower 10 causes the variator lens frame 9 and the compensator lens frame 17 to be guided by the guide bar 18 along a cam groove which is not shown but is provided in the cam ring 11. Then, zooming is effected with the position of these lens frames changed in the direction of the optical axis.
Further, there are provided a rail 20 for diaphragm blades 21 and 22; a relay lens holder 23; a relay lens frame 24; a main block 25 for an automatic focusing device; a spring 26; a base plate 27 for a light receiving element; a spacer 28; and a rotating shaft 29 for adjustment of the light receiving element.
FIG. 17 shows a relation between the amount of forward movement of the front lens 1 and a focusing distance (a distance between the focusing point of the front lens 1 moved forward, i.e., an in-focus point, and the front lens group 1). As shown, the amount of forward movement of front lens is approximately in proportion to the reciprocal of the focusing distance.
Therefore, the amount of forward movement of the front lens must be known for detecting the object distance R in connection with the drawback (1) mentioned in the foregoing. FIG. 18 is a front view showing an example of arrangement to meet this requirement. A brush 31 is secured by means of a screw 30 to the inner circumferential side of the front-lens frame 7. A pattern 32 is printed on the outer circumferential side of the stationary lens barrel 13. In a case where the position of the front lens is to be detected through a resistance value by employing a carbon resistor as the pattern 32, various causes for errors prevent the relation between the amount of forward movement of the front lens and the resistance value from being neatly expressed in a linear equation, as shown in FIG. 19. For example, there arises an error (Xk - Xo) relative to a resistance value ro. The acceptability of an encoder having this degree of accuracy depends on the degree of resolution of the encoder required for the automatic zooming function.
In addition to the variable resistance type method described above, it is also conceivable to detect the absolute position of the front lens by means of a brush 31 and a pattern 32 which are arranged as shown in FIG. 20. In this case, with the pattern 31 assumed to be consisting of an "n" number of pattern parts, the number of detectable areas are limited to 2.sup.n. Therefore, the acceptability of that arrangement also depends on the degree of resolution of the encoder required for the automatic zooming function.
Therefore, for a highly accurate automatic zooming function, it is necessary either to enhance the linearity of the resistance value of the carbon resistor or to increase the number of brushes. It is also necessary to compute the value R by obtaining the reciprocal of the result of detection.
In respect to the above-stated drawback (2), the focal length "f" is detected in the following manner: The position of the zoom ring is also detectable by the combination of a brush and a pattern like in the case of FIG. 18. FIG. 21 shows the detected value of the zoom ring as in relation to the focal length "f". Generally, an excessively large amount of movement of the lens relative to the unit rotational angle of the cam ring 11 brings about various problems such as unsmooth movement of the lens. Therefore, as shown in FIG. 7, the focal length "f" relative to a given rotational angle of the cam ring is arranged to change to a smaller degree at a wide-angle end position of the lens than at middle and telephoto-end positions. In order to find the focal length "f", therefore, a complex computing operation must be carried out by obtaining an approximate expression of the graph of FIG. 21.
In view of the foregoing, control by a CPU is performed as follows: The values R and "f" are first computed. Then, a required focal length "f" is computed according to the formula "f"=f (R, X). After that, a computing operation is again performed to obtain an encoder output value on the basis of the result of the computation. The position adjustment means is then controlled in such a way as to cause the encoder to produce its output at the above-stated value. Therefore, as described in respect of the drawback (3) in the foregoing, the computing operation of the CPU becomes complex.